Geocentric model says the Sun orbits around the Earth.

The geocentric model of the universe, in astronomy, is the theory that the Earth is at the center of the universe and the Sun and other objects go around it. Belief in the geocentric model was common in ancient Greece. The geocentric model was embraced by both Aristotle and Ptolemy, and most Greek philosophers assumed that the Sun, Moon, stars, and naked eye planets circle the Earth. Similar geocentric models were held in ancient China. Aristarchus of Samos proposed a heliocentric model of the Solar System.

Early printed rendition of a geocentric cosmological model.

The geocentric model was usually combined with a spherical Earth by ancient Greek and medieval philosophers. It is not the same as the older flat Earth model implied in some mythology. The ancient Greeks also believed that the motions of the planets were circular and not elliptical, a view that was not challenged in western culture before the 17th century.

The geocentric model held sway into the early modern age; from the late 16th century onward it was gradually replaced by the heliocentric model of Copernicus, Galileo and Kepler. Today, geocentric cosmology survives in the work of some creationist fundamentalist Protestant elements of Christianity, as well as literary treatments within alternate history science fiction.

The geocentric model: Classical Greece.

The geocentric model entered Greek astronomy and philosophy at an early point; it can be found in Pre-socratic philosophy. In the 6th century BC, Anaximander proposed a cosmology with the Earth shaped like a section of a pillar (a cylinder), held aloft at the center of everything. The Sun, Moon, and planets were holes in invisible wheels surrounding the Earth; through the holes, humans could see concealed fire. About the same time, the Pythagoreans thought that the Earth was a sphere, but not at the center; they believed that it was in motion around an unseen fire. Later these views were combined, so most educated Greeks from the 4th century BC on thought that the Earth was a sphere at the center of the universe.

In the 4th century BC, two influential Greek philosophers wrote works based on the geocentric model. These were Plato and his student Aristotle. According to Plato, the Earth was a sphere, at rest at the center of the universe. The stars and planets were carried around the Earth on spheres or circles, arranged in the order (outwards from the center): Moon, Sun, Venus, Mercury, Mars, Jupiter, Saturn, fixed stars. In the "Myth of Er," a section of the Republic, Plato describes the cosmos as the Spindle of Necessity, attended by the Sirens and turned by the three Fates. Eudoxus of Cnidus, who worked with Plato, developed a less mythical, more mathematical explanation of the planets' motion based on Plato's dictum stating that all phenomena in the heavens can be explained with uniform circular motion. Aristotle elaborated on Eudoxus' system. In the fully developed Aristotelian system, the spherical Earth is at the center of the universe. All heavenly bodies are attached to 56 concentric spheres which rotate around the Earth. (The number is very high because several transparent spheres are needed for each planet.) The Moon is on the innermost sphere. Thus it touches the realm of Earth, which contaminates it, causing the dark spots (macula) and the ability to go through Lunar phases. It is not perfect like the other heavenly bodies, which shine by their own light.

Adherence to the geocentric model stemmed largely from several important observations. First of all, if the Earth did move, then one ought to be able to observe the shifting of the fixed stars due to parallax. In short, the shapes of the Constellations should change considerably over the course of a year. In reality, the stars are so much further away than the Sun and the planets that this motion (which does exist) was not detected until the 19th century. The lack of any observable parallax was considered the death of any non-geocentric theory.

Another influencing observation was that Venus stays about the same brightness most of the time "and therefore is always about the same distance from Earth". In reality that is because the loss of light caused by its phases compensates for the increase in apparent size caused by its varying distance from Earth. Other objections included the idea, put forward by Aristotle, that the natural state of heavy objects like the Earth was at rest, and that some force was required to move them. It was also believed by some that the Earth's rotation on its axis would cause the air and objects in it (such as birds or clouds) to be left behind.

A major flaw in the Eudoxan and Aristotelian models based on concentric spheres was that they could not explain the changes in brightness of the planets caused by a change in distance.

Claudius Ptolemy and the geocentric model.

Although the basic tenets of Greek geocentrism were established by the time of Aristotle, the details of his system did not become standard. This honor was reserved for the Ptolemaic system, espoused by the Claudius Ptolemaeus of Alexandria, Egypt in the 2nd century AD. His main astronomical book, the Almagest, was the culmination of centuries of work by Greek astronomers; it was accepted for over a millennium as the correct cosmological model by European and Islamic astronomers. Because of its influence, the Ptolemaic system is sometimes considered identical with the geocentric model.

Ptolemaic system.

In the Ptolemaic system, each planet is moved by two or more spheres: one sphere is its deferent which is centered on the Earth, and the other sphere is the epicycle which is embedded in the deferent. The planet is embedded in the epicycle sphere. The deferent rotates around the Earth while the epicycle rotates within the deferent, causing the planet to move closer to and farther from Earth at different points in its orbit, and even to slow down, stop, and move backward (in retrograde motion). The epicycles of Venus and Mercury are always centered on a line between Earth and the Sun (Mercury being closer to Earth), which explains why they are always near it in the sky. The Ptolemaic order of spheres from Earth outward is:

Moon.

Mercury.

Venus.

Sun.

Mars.

Jupiter.

Saturn.

Fixed Stars.

The deferent-and-epicycle model had been used by Greek astronomers for centuries, as had the idea of the eccentric (a deferent which is slightly off-center from the Earth). In the illustration, the center of the deferent is not the Earth but X, making it eccentric (from the Latinex- or e- meaning "from," and centrum meaning "center"). Unfortunately, the system that was available in Ptolemy's time did not quite match observations, even though it was considerably improved over Aristotle's system. Sometimes the size of a planet's retrograde loop (most notably that of Mars) would be smaller, and sometimes larger. This prompted him to come up with the idea of an equant. The equant was a point near the center of a planet's orbit which, if you were to stand there and watch, the center of the planet's epicycle would always appear to move at the same speed. Therefore, the planet actually moved at different speeds when the epicycle was at different points on its deferent. By using an equant, Ptolemy claimed to keep motion which was uniform and circular, but many people did not like it because they did not think it was true to Plato's dictum of "uniform circular motion." The resultant system which eventually came to be widely accepted in the west was an unwieldy one to modern eyes; each planet required an epicycle revolving on a deferent, offset by an equant which was different for each planet. But it predicted various celestial motions, including the beginnings and ends of retrograde motion, fairly well at the time it was developed.

This drawing from an Icelandic manuscript dated around 1750 illustrates the geocentric model.

Geocentric model and the rival systems.

Not all Greeks agreed with the geocentric model. The Pythagorean system has already been mentioned; some Pythagoreans believed the Earth to be one of several planets going around a central fire. Hicetas and Ecphantus, two Pythagoreans of the 5th century BC, and Heraclides Ponticus in the 4th century BC, believed that the Earth rotated on its axis but remained at the center of the universe. Such a system still qualifies as geocentric. It was revived in the Middle Ages by Jean Buridan. Heraclides Ponticus is also sometimes said to have proposed that both Venus and Mercury went around the Sun rather than Earth, but the evidence for this claim is not clear. Martianus Capella definitely put Mercury and Venus on epicycles around the Sun.

Aristarchus of Samos was the most radical. He wrote a work, which has not survived, on heliocentrism, saying that the Sun was at the center of the universe, while the Earth and other planets revolved around it. His theory was not popular, and he had only one known follower, Seleucus of Seleucia.

Copernican system over geocentric model.

In 1543 the geocentric system met its first serious challenge with the publication of Copernicus's De revolutionibus orbium coelestium, which posited that the Earth and the other planets instead revolved around the Sun. The geocentric system was still held for many years afterwards, as at the time the Copernican system did not offer better predictions than the geocentric system, and it posed problems for both natural philosophy and scripture.

With the invention of the telescope in 1609, observations made primarily by Galileo Galilei (such as that Jupiter has moons) called into question some of the tenets of geocentrism but did not seriously threaten it.

Phases of Venus.

In December 1610, Galileo Galilei used his telescope to show that Venus went through phases, just like the Moon. This observation was incompatible with the Ptolemaic system.

Ptolemy placed Venus inside the sphere of the Sun (between the Sun and Mercury), but this was arbitrary; he could just as easily swapped Venus and Mercury and put them on the other side of the Sun, or made any other arrangement of Venus and Mercury, as long as they were always near a line running from the Earth through the Sun. In this case, if the Sun is the source of all the light, under the Ptolemaic system:

If Venus is between Earth and the Sun, the phase of Venus must always be crescent or all dark.

If Venus is beyond the Sun, the phase of Venus must always be gibbous or full.

But Galileo saw Venus at first small and full, and later large and crescent.

Astronomers of this time period saw the result of this being unsalvageable for a Ptolemaic cosmology, if the results were accepted as true. As a result, later 17th century competition between astronomical cosmologies focused on variations of Tycho Brahe's Tychonic system (in which the Earth was still at the center of the universe, and around it revolved the Sun, but all other planets revolved around the Sun in one massive set of epicycles), or variations on the Copernican system.

Gravitation: Newton and Einstein.

Johannes Kepler,after analysing Tycho Brahe's observations, constructed his three laws in 1609 and 1619, based on a heliocentric view where the planets moves in elliptical paths. Using these laws, he was the first astronomer to successfully predict a transit of Venus (for the year 1631).

In 1687, Isaac Newton devised his Law of universal gravitation, which introduced gravitation as the force that both kept the Earth and planets moving through the heavens and also kept the air from flying away, allowing scientists to quickly construct a plausible heliocentric model for the solar system.

In 1838, astronomer Friedrich Wilhelm Bessel successfully measured the parallax of the star 61 Cygni, disproving Ptolemy's argument that parallax motion could never be observed.

A geocentric frame is useful for many everyday activities and most laboratory experiments, but is a less felicitous choice for solar-system mechanics and space travel. While a heliocentric frame is most useful in those cases, galactic and extra-galactic astronomy is easier if the sun is treated as neither stationary nor the center of the universe, but rotating around the center of our galaxy.

Geocentric model today. Modern geocentrism.

Some religious fundamentalists still interpret their scripture as stating that the Earth is the physical center of the universe-known as modern geocentrism. Astrologers, while they may not believe in geocentrism as a principle, still employ the geocentric model in their calculations.

The contemporary Association for Biblical Astronomy, led by physicist Dr. Gerardus Bouw, holds to a modified version of the model of Tycho Brahe, which they call geocentricity.

In planetariums.

The geocentric (Ptolemaic) model of the solar system is still of interest to planetarium makers, as, for technical reasons, a Ptolemaic-type motion for the planet light apparatus has some advantages over a Copernican-type motion. The celestial sphere, used for teaching purposes and sometimes for navigation, is also still based on a geocentric system.

Geocentric model in science fiction.

As one might guess, alternate history science fiction has produced some literature of interest on the proposition that some alternate universes and Earths might indeed have laws of physics and cosmologies that are Ptolemaic and Aristotelian in design. This subcategory began with Philip Jose Farmer's short story, Sail On! Sail On! (1952), where Columbus has access to radio technology, and where his Spanish-financed exploratory and trade fleet sail off the edge of the (flat) world in his geocentric alternate universe in 1492, instead of discovering North America and South America.

Richard Garfinkle's Celestial Matters (1996) is set in a more elaborated geocentric cosmos, where Earth is divided by two contending factions, the Classical Greece-dominated Delian League and the (Chinese) Middle Kingdom, both of which are capable of flight within an alternate universe based on Ptolemaic astronomy, Aristotle's physics and Taoist thought. Unfortunately, both superpowers have been fighting a thousand-year war since the time of Alexander the Great.

References to the geocentric model.

Dreyer, J. L. E.. A History of Astronomy from Thales to Kepler. 2nd edition. New York: Dover Publications, 1953.

Evans, James. The History and Practice of Ancient Astronomy. New York: Oxford University Press, 1998.